This invention relates to processes to remove sulfur from petroleum feedstocks by utilizing solid sulfur sorbent of specified compositions.
Catalytic reforming processes play an integral role in upgrading straight-run or cracked naphtha feedstocks. These processes have become particularly important in recent years due to increased demand for low-lead and unleaded gasolines.
In a typical modern reforming process, a straight-run or cracked naphtha feed, having a boiling range from about 65.degree.-230.degree. C., or preferably 80.degree.-200.degree. C., is passed over a promoted noble metal catalyst on alumina at a temperature in the range of 400.degree.-550.degree. C., preferably 450.degree.-530.degree. C., a pressure in the range from atmospheric to 30 atmospheres, at a liquid hourly space velocity in the range 0.5-5, preferably 1-3, and a hydrogen to hydrocarbon mol ratio in the range 1-10. Variations in the conditions will depend in large measure upon the type of feed processed, such as whether the feed is aromatic, paraffinic or napthenic, and the desired increase in octane levels (see U.S. Pat. No. 4,082,697).
To achieve maximum run lengths and increased process efficiency, it is generally recognized that the sulfur content of the feedstock must be minimized to prevent poisoning of the catalyst. Preferably the feed will contain less than 2-10 parts per million by weight sulfur since the presence of much more sulfur in the feed decreases the activity, the stability and yield selectivity of the catalyst (see U.S. Pat. No. 3,415,737).
A common method of treating reformer feeds to reduce sulfur content is the hydrodesulfurization process wherein the naphtha is contacted with a sulfur-resistant hydrogenation catalyst in the presence of hydrogen. Catalysts for this service normally comprise Group VIB and/or Group VIII metals on refractory supports, such as molybdenum and cobalt metals, their oxides or sulfides, on alumina. The sulfur in the feed stream is converted to hydrogen sulfide, which may be separated from the naphtha by distillation, stripping and other conventional means, prior to reforming. Although good sulfur removal may be achieved by hydrodesulfurization units operating under severe conditions, the process is ultimately limited by the physical and chemical equilibrium concentration of hydrogen sulfide and mercaptans in the treated liquid feed.
If as little as 1.0 part per million by weight of H.sub.2 S remains in the naphtha feedstock to the reformer, it can build up to 2-3 ppm of H.sub.2 S in the reformer recycle hydrogen stream. Then the run length will be decreased by as much as 50%, C.sub.5 + yields will be decreased by 1.5-2.5 liquid volume percent, and hydrogen yields decreased 100-200 SCF/B. Thus, there exists a very strong economic incentive to remove substantially all sulfur from the reformer feed stream. The sulfur sorber unit containing sulfur sorbent provides the means for removing these trace amounts of H.sub.2 S and mercaptans and maintaining maximum performance of the catalytic reformer.
Sulfur sorbents can be prepared by mulling peptized alumina with a copper compound and forming particles of this mixture (see U.S. Pat. No. 4,224,191).
In general, sulfur sorbents must have high surface area and contain as much copper as possible. Carbon also has been used as a support (see U.S. Pat. No. 4,008,174).
It is known that certain clays are composed of fibrous crystalline structures rather than platy structures. Examples are attapulgite, sepiolite, halloysite and crysotile.
Sepiolite and attapulgite have been used as supports for catalytic metals for demetalation catalysts.
U.S. Pat. No. 4,196,102 discloses such a catalyst and U.S. Pat. No. 4,166,026 discloses a process using such a catalyst.
Attapulgite has been used to make catalytic sweetening catalysts for hydrocarbon oils. U.S. Pat. No. 2,361,651 discloses a catalyst with less than 5 weight percent copper, when copper is measured as the reduced metal, supported on attapulgite. U.S. Pat. No. 2,593,464 discloses a catalyst, to be used in the presence of oxygen, containing less than about 20 weight percent copper, measured as copper metal, and at least 8 percent water, supported on attapulgite clay. It has been discovered that copper can be comulled into fiberous clay supports to produce sulfur sorbers that have higher loadings of copper, good pore size, and can be made either with or without alumina added for structural strength.